petrosian was one of the best he played defensive and preyed on mistakes. he won the world championship twice first in 1963 v Botvinnik. and v Spassky in1966. and also won over 25 world class tournaments.
Best Defensive Chess Player

Petrosian was certainly one of the biggies. Ulf Andersson and Peter Leko are two more recent GMs who are known for their defensive skills.

Lasker was infamous for getting into worse positions and then finding the best practical chances and often capitalizing on them.

My choice is Peter Leko, the Drawcula of Hungary. In Leko's peak, Kasparov himself acknowledge his defensive prowess.

Seems that it depends whether you distinguish between a defensive player and someone who can defend well.
Anand is clearly brilliant defensively, but I don't think he's really a defensive player.

pfren, it doesn't say defensive style. Just says best defensive player. Defensive skill is what that implies, not defensive style. The ability to draw from bad positions. I would have to give that nod to Petrosian as well.
That said, if you strictly go based on defensive style, and draw ratio, gotta give the nod to Yusupov.

Some of the greatest defenders of all time: Lasker, Petrosian, Karpov, Korchnoi, Ulf Andersson, Kramnik, Aronian, Carlsen. Hikaru Nakamura is becoming well respected for his ability to defend tough positions too, but he's not in that group.
Of that group, my favorite players to study are Karpov and Kramnik. Carlsen is just amazing though.

Percentage of games lost over the course of their careers:
=1.Ulf Andersson 14.3%
=1.Alexander Khalifman 14.3%
3.Peter Leko 15.9%
=4.Tigran V. Petrosian 16.3%
=4.Boris Spassky 16.3%
6.Andrei Sokolov 16.9%
7.Vladimir Kramnik 17.6%
8.Borislav Ivkov 17.7%
=9.Vassily Smyslov 17.9%
=9.Ratmir Kholmov 17.9%
Karpov, Kasparov, Anand, Yusupov, Aronian, Tal, Korchnoi, Carlsen, Nakamura and Lasker have all lost more.

Bobby Fischer has to be ranked as one of the all time defensive players. Fischer would usually take an offered pawn and hold on for dear life. There are many instances of Fischer walking into prepared traps and busting them over the board. The common conception of Fischer is an attacker, and Fischer certainly would go on the offense if there was a clear opportunity but he hated obscure tactics and his attacks were almost always sound. Fischer hated draws so he lost more games than some of the other defensive players. I understand I am departing from the OP definition of defensive player as one who plays for a draw, but to me a great defensive player is one who is willing to take on difficult positions.

Both Nakamura and Fischer are uncompromising players who take a lot of chances to win, and so they end up losing quite a few games. Of the American players, Boris Gulko and Samuel Reshevsky were probably the hardest to beat.
nameno1had - Karpov came in at 18.0%.

Percentage of games lost over the course of their careers:
=1.Ulf Andersson 14.3%
=1.Alexander Khalifman 14.3%
3.Peter Leko 15.9%
=4.Tigran V. Petrosian 16.3%
=4.Boris Spassky 16.3%
6.Andrei Sokolov 16.9%
7.Vladimir Kramnik 17.6%
8.Borislav Ivkov 17.7%
=9.Vassily Smyslov 17.9%
=9.Ratmir Kholmov 17.9%
Karpov, Kasparov, Anand, Yusupov, Aronian, Tal, Korchnoi, Carlsen, Nakamura and Lasker have all lost more.
Kasparov, according to chessgames +788 -123 =775
7.2% losses...
Who is your choice for best defensive chess player? Who draws easily both with white and black.